The present invention relates to a method for manufacturing a printed wiring board and, more particularly, is directed to a method for the formation of a solder land.
Conventionally, a solder resist coating is formed for the purpose of preventing the adhesion of solder onto undesired portions of a wiring circuit having a desired pattern. The solder resist film is formed to reduce the volume of solder used, prevent a short circuit from arising due to a soldering bridge, form an appropriate soldering fillet, and protect conductors on the wiring circuit from the external environment.
To form such a solder resist coating, a screen printing method and a photographic developing method is used. The screen printing method is mainly employed for a non-throughhole wiring board, while the photographic developing method tends to be used for a through-hole wiring board (including multi-ply board).
The reason is that the through-hole wiring board has a more miniaturized circuit as compared to the non-throughhole wiring board, thus leading to a miniaturization of the solder resist pattern, which is better accomplished by the photographic developing method due to its adaptation for such circumstances.
Nevertheless, the photographic developing method is poor in productivity and high in production costs owing to increased raw material cost, and hence it s unsuitable for the non-through hole wiring board which requires a mass-productivity and a lower cost.
Also, the solder resist coating in accordance with the screen printing method is executed after etching in a subtractive method (etched wheel method ) as shown in FIGS. 8 to 11. That is, an etching resist 3 corresponding to a predetermined pattern is applied on a copper foil 2 of a copper laminated board 1 which has been subjected to a cutting process shown in FIG. 8 FIG. 9, and then a wiring circuit 5 having a predetermined pattern is formed on the front of the insulating board 4 by means of predetermined etching process (FIG. 10).
Afterward, a solder resist 6 is printed on the wiring circuit 5 in FIG. 10 through a predetermined solder mask as shown in FIG. 11 .
The conventional method of forming solder resist 6 has, however, a limitation in accurately forming it after forming the miniaturized circuit. Therefore, as shown in FIGS. 12 and 13, particularly for the formation of a solder land 7 among the solder resist 6 in the process of FIG. 11 the printing may be made in compliance with the required specification of a solder mask diameter in the first printing step, and then a solder land 8 may be printed in a slightly larger diameter (for example, +0.1 m/m to 0.2 m/m in diameter) than the solder mask diameter in the second printing step, followed by the finish over the entire surface. Or alternatively, the solder resist printing is performed in the first printing step as shown in FIG. 14, and then the solder land portion of the solder resist 6 is trimmed 9 by character ink in the second printing step as shown in FIG. 15 (Note that steps shown in FIGS. 14 and 15 may be performed in reverse order).
The following are three main requirements of the printing technique at the time of the solder resist formation in the conventional method of manufacturing the printed wiring board described above.
(1) The copper foil portion of the wiring circuit to be covered with the solder resist must not be exposed.
(2) The area of the solder land must be secured in compliance with the design dimensions so as to suppress blurring of the solder resist.
(3) Minimum coating thickness of the solder resist must be maintained.
In order to satisfy the above requirements of the screen printing method, the requirements (1) and (2) are, however, contrary to each other, which leads to a limitation on miniaturization of the solder land by the screen printing method.
That is, in order to execute the requirement (1), it must be recognized that the printing surface of the solder resist 6 remains uneven due to the existence of the wiring circuit 5 after etching during the manufacture of the printed wiring board as shown in FIG. 16. Upon printing of the solder resist 6, the solder resist ink must be deposited into the recess 12 by applying a printing pressure on a squeegee 11 when printing the solder resist 6 on the front of the wiring circuit 5 through the screen 10 as shown in FIG. 17.
Thus, in principle, the higher the printing pressure and the lower the ink viscosity, the better the solder resist ink flows into the recess 12.
Next, in order to suppress blurring which is the second requirement, the printing pressure of the squeegee 11 must be kept as low as possible.
That is, as shown in FIG. 18, a higher printing pressure causes the top 11a of the squeegee 11 to bend, which results in a larger area of the top 11a in contact with the printed surface 13 of a matter to be printed 14 (Refer to FIG. 19), thereby bringing about a poor separation from the plate, to consequently lead to the occurrence of the blur.
Although apparent from the principle of the screen printing, the screen 10 which has completed the printing for some area must be promptly separated from the printed surface 13. However, if the top 11a of the squeegee 11 has a larger contact area, it takes the screen 10 more time to separate from the printed surface 13, which leads to a poor separation from the plate. In order to prevent this phenomenon and obtain screen printing without blurring, the printed surface must be impressed sequently from the front thereof to the rear through the medium and then separated from the sheet from the front to the rear through the medium without impressing the stamp surface onto the sheet at one time. Also, this is the same where blurring occurs when the entire surface is impressed on the sheet at one time and a transverse minute vibration (a minute vibration following progression of the squeegee) is applied due to an unsteady manual process.
Therefore, as the printing pressure of the squeegee 11 becomes lower, better printing accuracy and without blur using a higher circuit density can be achieved. FIGS. 20 and 21 show a state of the printed surface 13 when squeegee 11 is under a lower printing pressure.
The following conclusion is derived from the above discussion. The conditions of the screen printing technique for the requirements of the perfect covering of the first copper foil portion which is required in forming of the solder resist by means of the screen printing method are that preferably the printing pressure of the squeegee 11 is high and the viscosity of the solder resist ink is low. While on the contrary, the conditions of the screen printing technique for the requirement that the blur of the second solder resist must be suppressed are that preferably the printing pressure is low and the viscosity of the solder resist ink is high. Under these inconsistent conditions conventionally, the screen printing method must be carried out having only one or two minutes between the movements of the squeegee 11.
Thus, even the adoption of the methods as shown in FIGS. 12, and 13, and FIGS. 14 and 15 in forming of the solder resist does not basically change the state of unevenness of the printed surface. Due to the contradictory relationship described above, there is a technical limitation for mass production that the diameter of the solder land 7 formed by the application of solder resist 6 is limited to about 1.4 mm.
Furthermore, with regard to controlling the thickness of the resist coating which is referred to as the third technical requirement on forming the solder resist by the screen printing method, the thickness of the coating of the solder resist 6 conventionally provided on the front of the wiring circuit 5 is shown as portion A in FIG. 22. The portion B is about half of the portion A in the coating thickness, and the difference in coating thickness obtained of portions A and B is conventionally inevitable resulting in problems, such as resistance to moisture, at the portion B during the environmental test or the like. It is therefore very advantageous to entirely equalize the coating thickness, which is not possible using the conventional methods.
Thus, the present invention was conceived in view of the problems in forming the solder resist by means of such conventional screen printing method. An object of the present invention is to provide a method of forming the solder resist by means of the screen printing method, which is capable of forming a minute solder land in a high-density wiring circuit.